Verizon said a shortage of iPhone 5S devices might have contributed to fewer new customers in the third quarter.

The wireless carrier added 927,000 subscribers through contracts – more valuable than prepaid customers — a large figure but a 40% drop from the 1.54 million it added in the same period last year.

“Obviously we could have done more,” Fran Shammo, Verizon’s chief financial officer, said during the earnings conference call. There was tight supply, “especially around the 5S and I think that constrained us for this quarter.”

In an interview, Shammo said the iPhone 5S is selling far better than the 5C, a less-pricey model that comes in five colors. “The 5S was constrained. The 5C, not so much,” he said.

Two models with different colors made it difficult to manage the supply chain, particularly with 5S demand higher than expected and 5C demand likely lower than expected, he said. “So it creates a bit of a problem in the supply chain.”

Apple didn’t respond to requests for comment.

Verizon didn’t estimate how many subscribers it could have added if there wasn’t a dearth of the more popular 5S model. Shammo said last year’s bulge in subscriber additions – 1.54 million in the third quarter and 2.1 million in the fourth – was in part because it was the first time Apple sold an iPhone that worked on faster networks known as LTE, and the first time Verizon offered an older iPhone model free. “You don’t have that repeating this year,” he said.

Apple’s dual iPhone strategy is facing challenges. Apple told two assemblers earlier this month that it would reduce orders of the iPhone 5C, The Wall Street Journal reported. Gervais Pellissier, chief financial officer of the French telecom company Orange, said the iPhone 5C is selling much less than the 5S. The 5C isn’t doing as well because it is too expensive, he said.

Shammo expects a bump in subscriber additions in the fourth quarter as more iPhone 5S supply becomes available. The “backlog at the end of September” will spill over into next quarter, he said.

Overall, Verizon activated about 3.9 million iPhones this quarter, up from 3.1 million during the same period last year. The percentage of smartphone activations that were iPhones has increased to 51% from 45% a year earlier.